1. Field of the Invention
The present invention relates generally to production of bottles by extrusion blow molding and, more particularly, to a method and apparatus for forming a neck finish in a wide range of plastic water dispenser bottles that has a smooth, straight inner diameter and is fully calibrated, not requiring any post trimming.
2. Description of the Related Art
Containers and other hollow articles are frequently formed by blow molding. The blow molding process involves providing a tube of heated and softened plastics material (parison) about which a two-piece mold is closed. Air or another gas is introduced under pressure into the parison to expand it against the walls of the mold, forming the desired hollow article. Such blow molding machines can be of various types, the most common of which are extrusion-blow molding machines and injection-blow molding machines.
A requirement for the large bottles produced for the water bottle industry is a design that allows effective cleaning so that the bottles can be refilled and reused in a drinking water dispenser. An important feature with respect to the cleaning is a smooth, straight inner diameter for the neck of the bottle, so that there are no irregular surfaces that could retain bacteria or foreign matter. To ensure the desired surface for the inner diameter of the neck, the preferred method of the prior art has been to injection mold the desired neck geometry into a bottle preform that is subsequently reheated and blown to the final shape; i.e., form the bottles by the injection-blow molding process. While this process is very effective in producing bottles having the desired features, it tends to be relatively expensive due to the additional equipment and time associated with injection molding step.
Since extrusion-blow molding machines generally produce a less expensive container, they have also been used to some extent to form water bottles. However, they have not been well accepted because the inner diameter of the bottle neck lacks the desired smooth, straight geometry due primarily to variations in the geometry of the outer diameter. In extrusion-blow molding machines, plastics material is heated and softened in an extruder and is conveyed into a die head from which a tubular parison is extruded. The parison can either be continuous, whereby a plurality of molds are sequentially enclosed about successive, axially spaced portions of the parison, or individual parisons can be intermittently extruded into the mold area using an accumulator. In either case, the material is then ejected through an annular die to form a parison having the desired length, diameter, and wall thickness distribution.
More specifically, in the extrusion-blow molding process, the mold is closed over the parison pinching off (closing) the ends of the tubular parison. Air or another gas is injected inside the parison through a blow pin, expanding the parison to match the mold cavity. Obviously, the mold in this process controls only the shape of the outer surface of the bottle, including the neck. Since the outer surface of the neck has an irregular contour to facilitate use in dispensers or water coolers, the corresponding inner surface is also irregular since the material forming the inside wall is not constrained, resulting in varying wall thickness that further distorts as the bottle cools to ambient temperature.